After just five professional fights, Williams's quest for boxing credibility will reach a defining point when he takes on the South African in Brisbane on February 8.

Troublesome Wallabies five-eighth Quade Cooper will make his boxing debut on the undercard.

A veteran of 60 pro fights and having been in the ring with Mike Tyson, Lennox Lewis and Evander Holyfield, Botha is a risky opponent for Williams at this stage of his career. Victory against the ''White Buffalo'' for the WBA belt would propel his name internationally, with the fight to be broadcast in New Zealand, Australia and South Africa.

But the dual-code superstar told Fairfax Media that defeat would see him throw in the towel and concentrate on his return to the NRL with the Sydney Roosters next month.

''If I get knocked out then that will probably be the end,'' Williams said in his Auckland hotel room on Saturday. ''Right now I can see myself doing boxing full-time in the future, but for that to come to fruition I need to fight guys like this and beat them, otherwise it's a waste of time. Some boxers go their whole lives building a record until people want to watch them. I've had to jump really quickly. This fight will be defining. It will tell me if I can keep going. I could easily keep fighting on the undercards, but you've got to take some risks. I'm not getting any younger. If this doesn't go too well, I could just stay in one sport. If I lose I will definitely be upset because I know it will probably be the end of it. If I ever got knocked out I don't think my mum would let me back in the ring, but at least I had a crack. And if I win then it opens doors.''

Those bold statements give an insight into how seriously the 27-year-old is taking his next step in the ring. It's all or nothing.

At first, boxing was a trial for Williams. He started with no amateur background and for his first three fights, learnt his craft from YouTube footage. ''At the start it was an experiment,'' the 27-year-old admitted. ''I didn't see a future in it until my last fight [against Clarence Tillman]. It was a bit of a reality shock when I first got a boxing trainer.

''I saw how much I improved with a boxing trainer and my confidence shot up from there. Now I'm definitely keen to try doing this for a living to see how far I could go.''

Against Botha, Williams is expected to play the long game in the scheduled 12-rounds, work the jab and back his reach to keep his distance. The 44-year-old Botha will come out blazing and try to end it early. If Williams can use the ring to his advantage, he should have too much fitness and athleticism.

''I wouldn't be in the fight game if I was the type of fighter that stood there and brawled the whole time,'' Williams said. ''Those guys have got too much to lose. I enjoy it, but if I cop a few too many blows … I don't want to be a bit punchy when I'm older. I want to spend time with my kids.''

Williams also said he needed to control his nerves. ''I'm so nervous that I want to be a perfectionist,'' he said. ''The emotions you feel are intense. That's one of the biggest things about boxing; I'm so scared before every fight, but that fear keeps me on edge. I used to be like that with footy.''